Clinical evidence suggests that hearing loss during infancy and childhood increases the probability of language and learning disabilities. Experimental data suggest that such communicative disorders are due in part to changes in auditory system anatomy and function, induced by lack of acoustic experience during development. These considerations indicate the importance of understanding auditory development, and the degree to which experience can influence its course. Developmental studies can also provide insight into the function of the adult auditory system. The proposed research will investigate several current problems in normal auditory development, including: the afferent and efferent innervation of the organ of Corti; the growth of neurons and synapses in the cochlear nuclear complex; levels of neural growth factors in the developing inner ear and cochlear nucleus; and early neural function in the VIIIth nerve and cochlear nucleus. The neural plasticity of auditory development will also be studied in a series of experiments. The effects of profound acoustic deprivation during auditory development upon the function of the auditory system will be assessed. Acoustic deprivation will include surgical removal of the middle ear transmission apparatus to provide approximately 60 dB of isolation from air-conducted, and 40 dB of isolation from bone-conducted, stimuli. A sound- attenuated rearing environment will provide an additional 80 dB of isolation from external sounds. Mechanical displacement of the stapes with a piezoelectric driver is used to restore auditory function. Also, tetrodotoxin blockade of neural transmission will be used to delay the normal onset of normal auditory function. The effects of this procedure on VIIIth nerve fiber and cochlear nucleus firing patterns will be evaluated. In addition, animals will be reared in an environment restricted to a 1/3 octave band of noise. Their innervation patterns will be assessed in adulthood to determine whether experience produces preferential representation in the central auditory pathway. The influence of neonatal lesions of cochlea on the development of the olivocochlear efferents will be studied, and the effects of manipulation of growth factors on the neural development of the cochlea and cochlear nucleus will also be evaluated.